Polymer coatings for enhanced and field-repairable transparent armor

ABSTRACT

A coating of atactic polypropylene over a transparent armor substrate improves resistance to penetration while allowing convenient repair of minor abrasions and scratches.

BACKGROUND

Drawbacks to conventional transparent armor include the need to usethicker panels to achieve desired levels of protection, thus incurring aweight penalty, and environmental erosion and scratching of the surface,which reduces transparency. A need exists to mitigate these problems.

BRIEF SUMMARY

In a first embodiment, an armor system includes a hard, transparentarmor substrate, and a transparent coating of atactic polypropylenebonded to the armor substrate.

In another embodiment, a vehicle incorporates the armor system of thefirst embodiment, with the transparent coating configured to face anexterior surface of the vehicle, the armor system configured as awindow, windscreen, or viewing port of said vehicle.

A further embodiment involves treating the armor system of the firstembodiment by heating and smoothing the transparent coating, therebyimproving optical clarity thereof.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the increase in velocity required to penetrate armor (V-50)due to the presence of a 19 mm polyurea coating.

DETAILED DESCRIPTION

Definitions

Before describing the present invention in detail, it is to beunderstood that the terminology used in the specification is for thepurpose of describing particular embodiments, and is not necessarilyintended to be limiting. Although many methods, structures and materialssimilar, modified, or equivalent to those described herein can be usedin the practice of the present invention without undue experimentation,the preferred methods, structures and materials are described herein. Indescribing and claiming the present invention, the following terminologywill be used in accordance with the definitions set out below.

As used in this specification and the appended claims, the singularforms “a”, “an,” and “the” do not preclude plural referents, unless thecontent clearly dictates otherwise.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

As used herein, the term “about” when used in conjunction with a statednumerical value or range denotes somewhat more or somewhat less than thestated value or range, to within a range of ±10% of that stated.

As used herein, the term “armor substrate” refers to new andconventional forms of transparent armor including, without limitation,laminates of soda-lime or borosilicate glass with polycarbonate as wellas transparent ceramic armor including aluminum oxynitride (“Alon”),spinel (including nanocrystalline spinel), and the like, andcombinations thereof.

As used herein, “atactic polymer” refers to a polymer composed ofatactic macromolecules, which are regular macromolecules in which theconfigurational (base) units are not all identical.

Description

Elastomeric coatings were found to substantially increase the ballisticlimit of underlying steel armor substrates when applied to the outsidesurface (that is, the “strike-face”) with a composite array ofelastomer-steel panels enjoying increases armor penetration resistance,as reported in Roland et al., “Elastomer-steel laminate armor” CompositeStructures 92 (2010) 1059-1064, incorporated herein by reference.

Various coatings including polyurea and butyl rubber have shown tofunction well in this application, and the coating itself may include acombination of materials.

FIG. 1 shows the increase in average velocity required to penetratearmor (V-50) due to the presence of a 19 mm polyurea coating. Thecoating contribution to penetration resistance systematically increaseswith increasing substrate hardness. On steel substrates, massefficiencies exceeding a factor of two have been achieved.

With regard to conventional transparent armor, thicker panels arerequired to achieve higher ballistic performance, with a concomitantweight penalty which is especially undesirably in the case of vehicles,adversely impacting performance, fuel economy, and payload, while thebulkier panels impinge on interior space. Furthermore, conventionaltransparent armor can be prone to environmental abrasion or scratching,reducing transparency and requiring costly and time-consuming repair.

This armor system may be applied to vehicles including manned orunmanned vehicles suitable for travel on the ground, or in the air, onthe surface of water or underwater, and combinations thereof. It may beused in windows, windscreens, viewing ports, and the like.

As described herein, a transparent armor system includes a polymercoating applied to a transparent armor substrate. The density by area ofthis transparent armor system can be less than that of conventionalarmor systems while providing equal or greater protection.

The protective function of the coating is believed to arise from animpact-induced phase transition with consequent large energy absorption,so that the substrate should be stiff enough to allow rapid compressionof the coating. Atactic polypropylene with a glass transitiontemperature of about −20° C. functions as a suitable coating due to thisphenomenon, while providing the desired transparency.

Armor Substrate

The armor substrate is preferably transparent and with sufficientrigidity and hardness to support the coating while also itself resistingpenetration. Most preferably, the armor substrate has a hardness of atleast 150, 200, 300, 400, 500 , or more, as measured using the Brinellmethod with a tungsten ball of 10 mm diameter and 3,000 kg force.

The armor substrate may be one or more new or conventional forms oftransparent armor including, without limitation, laminates of soda-limeor borosilicate glass with polycarbonate and transparent ceramic armorincluding aluminum oxynitride (“Alon”), spinel (includingnanocrystalline spinel), and the like, and combinations thereof.Nanocrystalline ceramic material that might be suitable for use as anarmor substrate is described in commonly-owned U.S. Provisional PatentApplication No. 61/907,440 filed on Nov. 22, 2013, incorporated hereinby reference.

Traditional bullet-resistant glass is available with coatings under thetrade names MARGARD and MAKROLON intended to improve scratch resistance.The present armor system may be used with any such forms of coatedtransparent substrates, termed secondary coatings to distinguish themfrom the atactic polypropylene coating of the invention. It is believedthat hard coatings may increase the effective hardness of the glass,thus improving performance of the system as seen in FIG. 1. Thepolypropylene coating senses the hardness of the substrate oflength-scales commensurate with the wavelength of the longitudinalpressure wave—this may guide the design of the thickness of a secondarycoating.

Coating

The polymeric coating is preferably atactic polypropylene. It was foundthat isotactic polypropylene would crystallize and fail to provide thedesired ballistic performance. A suitable molecular weight may be fromabout 40 to about 80 kilograms/mol for an atactic polypropylene polymer.In preparing the polymer, it should be cooled quickly to avoid formationof crystals large enough to scatter visible light.

The coating thickness may range, for example, from about 0.25 cm toabout 2.0 cm.

The coating may be bonded to the armor substrate using varioustechniques. It may be in direct contact with the armor substrate orbonded thereto via an intermediate adhesive. It may be cast into placeon the armor substrate. Mechanical bonding may be used, for exampleusing a frame, clamps, bolts, or other fasteners. A combination ofbonding techniques may be used.

An advantage of this transparent polymeric coating is its reversiblesolidification (as opposed to solidification via a practicallyirreversible chemical change in other polymers). Thus, abrasions andscratches may be removed by heating, optionally while contacting thesurface of the polymer with a smooth surface. It was found that atemperature of about 100° C. was sufficient to repair atacticpolypropylene. Such repairs could easily be made in the field.

Concluding Remarks

All documents mentioned herein are hereby incorporated by reference forthe purpose of disclosing and describing the particular materials andmethodologies for which the document was cited.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without departing from the spiritand scope of the invention. Terminology used herein should not beconstrued as being “means-plus-function” language unless the term“means” is expressly used in association therewith.

What is claimed is:
 1. A transparent armor system comprising: a hard,transparent armor substrate, and a transparent coating consisting ofatactic polypropylene having a thickness of from about 0.25 cm to about2.0 cm bonded to the armor substrate as an outside surface.
 2. Thetransparent armor system of claim 1, wherein said armor substrate isselected from the group consisting of (1) laminates of soda-lime orborosilicate glass with polycarbonate and (2) transparent ceramic armor.3. The transparent armor system of claim 1, wherein said armor substrateis aluminum oxynitride or spinel.
 4. The transparent armor system ofclaim 1, wherein said armor substrate has a hardness of at least 150Brinell as measured with a tungsten ball of 10 mm diameter and 3,000 kgforce.
 5. The transparent armor system of claim 1, further comprising asecondary coating with a hardness greater than that of the armorsubstrate, disposed between the armor substrate and the transparentcoating of atactic polypropylene.
 6. The transparent armor system ofclaim 1, wherein said transparent coating is bonded to said armorsubstrate mechanically and/or with an adhesive.
 7. A vehicle comprisinga transparent armor comprising: a hard, transparent armor substrate, anda transparent coating consisting of atactic polypropylene bonded to thearmor substrate having a thickness from about 0.25 cm to about 2.0 cm,configured to face an exterior surface of said vehicle as an outsidesurface, wherein said transparent armor system is configured as awindow, windscreen, or viewing port of said vehicle.
 8. A method oftreating a transparent armor system, the method comprising: (a)providing an armor system comprising: a hard, transparent armorsubstrate, and a transparent coating consisting of atactic polypropylenehaving a thickness from about 0.25 cm to about 2.0 cm bonded to thearmor substrate as an outside surface; and (b) heating and smoothing thetransparent coating, thereby improving optical clarity thereof.
 9. Themethod of claim 8, wherein said heating and smoothing comprisescontacting with said transparent coating with a smooth surface.